External Discharge Hall Thruster

ABSTRACT

The invention is a Hall thruster that does not have any discharge channel, and magnetic pole piece. The Hall thruster utilizes permanent magnets to produce magnetic field with strong radial component in front of an annular anode, and expands propellant directly into vacuum through the anode acting also as a gas distributor. The invention reduces mass and complexity of conventional Hall thrusters, and offers a radical solution to discharge channel and magnetic pole piece erosion problem.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional Application Ser. No. 62/277,992 filed Jan. 13, 2016, the disclosure of which is incorporated herein.

FIELD OF THE INVENTION

The present invention relates to space vehicle engines in general, and to Hall thrusters in particular.

BRIEF DESCRIPTION OF THE DRAWINGS

Features of the present invention can be better understood with reference to the drawings described below. The drawings are not necessarily to scale; emphasis instead generally being placed upon illustrating the principles of the invention. Cathode is not shown.

FIG. 1 illustrates a cutaway view of a preferred embodiment of the present invention.

FIG. 2 schematically illustrates a magnetic field configuration calculated for a preferred embodiment of the invention.

FIG. 3 illustrates a cutaway view of another preferred embodiment of the present invention.

FIG. 4 schematically illustrates a magnetic field configuration calculated for another preferred embodiment of the invention.

BACKGROUND OF THE INVENTION

Hall thruster is a type of electrostatic electric propulsion device, and it is used to carry out spacecraft attitude and orbit maneuvers.

Small satellite market is large, and growing due to low cost and short development time. Hall thrusters have high thrust density/efficiency, reliable/robust operation, simple design, and extensive space flight heritage. These main features make them attractive candidates for small/micro satellite applications. Scaling-down these devices to low power levels is quite a challenge because of the large surface area to volume, and the physical limits to the magnetic circuit miniaturization. Besides, scaling-down the thruster channel geometry results in short lifetime because of rapid erosion of thruster parts like discharge channel and magnetic pole piece, caused by increased plasma-wall interactions.

Plasma-wall interactions have substantial effects on discharge characteristics as well as on thruster performance. Power loss to the channel walls is the main power/efficiency loss source, and cause of erosion in conventional Hall thrusters. Therefore, development of low power Hall thrusters with longer lifetime remains an active area of research.

SUMMARY OF THE INVENTION

The present invention is a new kind of Hall thruster, which offers a radical solution to the above-mentioned problem of Hall thrusters by producing and sustaining plasma discharge completely outside a cavity. According to one aspect, the invention features a Hall thruster without discharge channel walls, magnetic pole pieces, and electromagnetic coils.

The invention uses permanent magnets enabling power reduction relative to thrusters with electromagnets.

It is an object of the present invention to reduce overall propulsion system dry mass/volume, complexity, and cost compared to conventional Hall thrusters.

It is another object of the present invention to provide a Hall thruster capable of a very high lifetime with comparable thrust-to-power ratio, specific impulse, and anode efficiency.

Features and advantages of the invention will become more apparent from the following description and from the claims.

DETAILED DESCRIPTION OF THE INVENTION

Hall thrusters comprise an annular discharge channel (i.e., stationary plasma thruster and thruster with anode layer) or a cylindrical discharge channel (i.e., cylindrical hall thruster). Radial magnetic field produced using a magnetic circuit having magnetic pole pieces, inner/outer electromagnetic coils, inner/outer magnetic screens, and inner/outer magnetic cores. The present invention eliminates any kind of discharge channel walls, and utilizes only permanent magnets as a magnetic circuit. The radial magnetic field is substantially higher compared to conventional Hall thrusters. The magnetic circuit supports a very strong confinement of electrons increasing electron neutral collision frequency to produce ion beam current. This is the reason why the invention does not require a cavity to form plasma.

In the present invention, an axial electric field is established between a positively biased anode, and a cathode. Working gas injected through the anode gets ionized due to collisions with energetic electrons. Strong radial magnetic field impedes plasma electrons from directly reaching the anode, and supply a potential drop in the axial direction. Ion acceleration due to the potential difference generates thrust.

The invention neither has a discharge channel wall nor a magnetic pole piece. Therefore, it is free from channel wall erosion problem. Some of the anode's surface is extended outside the magnetic circuit.

Design of the invention may be understood with reference to FIG. 1 and FIG. 3. A preferred embodiment of the invention comprises a positively biased electrode anode 1 also serving as propellant distributor, permanent magnets 2, heat insulation walls 3, front walls 4, thruster body 5, gas distributor 6, tube fitting 7 and a cathode, which is not shown.

Magnetic field profile of the invention may be understood with reference to FIG. 2 and FIG. 4

The invention is distinguished in these aspects:

1) There is no discharge channel walls, and thus ionization of neutrals and acceleration of ions occur in vacuum.

2) Radial magnetic field strength is uncommonly high.

3) It does not have any magnetic pole piece, magnetic core, magnetic screen, or electromagnetic coil, which are the heaviest component of Hall thrusters.

4) It offers additional freedom of design optimization to make extremely non-uniform propellant feed or uniform easily by changing the anode 1 design.

5) An embodiment of the invention comprises a tube fitting 7, which is concentrically aligned with the anode delivers ionizable gas to the gas distributor 6 in order to improve azimuthal uniformity of the ionizable gas.

An embodiment of the invention has been tested experimentally, and results are given in reference: Karadag B., Cho S., Oshio Y., Funaki I., and Komurasaki K.: Preliminary Investigation of an External Discharge Plasma, 52nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, Jul. 25-27, 2016, Salt Lake City, Utah.

As used in this specification and the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the content clearly dictates otherwise. The term “plurality” include two or more referents unless the content clearly dictates the otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains.

It should be noted that the description of the embodiments and attached figures presented in this specification does not limit the scope of the invention as covered by the appended claims, and serves only for a better understanding of the invention.

It is to be understood that a person of ordinary skilled in the art can make adjustments or amendments to the attached figures and above described embodiments that would still be covered by the following claims. 

What is claimed is:
 1. A Hall thruster comprising: a magnetic circuit comprising inner and outer permanent cylindrical hollow magnets concentrically aligned; an annular anode located inside the magnetic circuit, extending outside the magnetic circuit, and having plurality of holes or slots arranged azimuthally; an inner and/or outer annular insulation walls placed between the anode and the magnetic circuit; a gas distributor adjacent to the anode to inject an ionizable gas; a cathode near the anode, the anode and cathode configured to generate an electric field;
 2. A Hall thruster as recited in claim 1, wherein a tube fitting delivering ionizable gas to the gas distributor is concentrically aligned with the anode;
 3. A Hall thruster as recited in claim 1, wherein peak radial magnetic field strength within the anode holes or slots or cavity is larger than 600 Gauss.
 4. A Hall thruster as recited in claim 1, wherein the magnetic circuit comprises permanent magnets arranged azimuthally to produce magnetic field lines similar to cylindrical hollow magnets.
 5. A Hall thruster as recited in claim 1, wherein heat insulation walls are partly or fully removed.
 6. A Hall thruster as recited in claim 1, wherein a planar wall, having plurality of holes or slots arranged azimuthally is placed between the rear surface of the anode and the front surface of the magnetic circuit, and separates the anode completely from the permanent magnets and thruster body.
 7. The Hall thruster of claim 1, wherein the ionizable gas is a gas selected from the group of gases consisting of xenon, krypton, and argon.
 8. The Hall thruster of claim 1, wherein the ionizable gas comprises a vapor produced from an element selected from the group of elements consisting of bismuth, iodine, magnesium, and zinc.
 9. The Hall thruster of claim 1, wherein the ionizable gas comprises nitrogen and oxygen.
 10. The Hall thruster of claim 1, wherein the ionizable gas comprises carbon dioxide.
 11. A Hall thruster substantially as described in the above specification and attached figures. 